The present invention relates to a photomask design method for the manufacture of a semiconductor device and, more particularly, to a method of designing a photomask such as a Levenson phase shift mask in which a phase shifter is formed of one of adjacent patterns. The present invention also relates to a photomask design apparatus and a recording medium storing a program for realizing photomask design with a computer.
This application is based on Japanese Patent Application No. 9-71929, filed Mar. 25, 1997 and Japanese Patent Application No. 9-267346, filed Sep. 30, 1997, the content of which is incorporated herein by reference.
The following techniques are known as conventional Levenson phase shift mask design methods.
First prior art: "Automatic Pattern Generation System for Phase Shifting Mask", Symposium on VLSI Technology, JSAPCAT, No. AP911210 (1991), pp. 95-96.
This paper examines an automatic shifter arranging method. In automatically assigning the phases of light passing through patterns, an arbitrary pattern is selected first, and a phase of 0.degree. is assigned for the selected pattern. For the next arbitrary pattern, a phase opposite to that of a pattern near the next selected pattern, whose phase is already assigned and which has a longest side opposing the next pattern, is assigned. If there are a plurality of patterns whose longest sides oppose the next pattern, and they have different phases, a warning is issued, and processing is stopped.
Second prior art: Andrew. R. Neureuther et al., "Investigating Phase-shifting Mask Layout Issues Using a CAD Toolkit", IEDM Tech. Digest, 1991, pp. 705-708.
This paper explains an apparatus which automatically arranges a shifter for a given design layout which is reduced based on the reduction magnification determined by the designer, and if the automatic shifter arrangement has an inconsistent portion, displays that portion.
Third prior art: Japanese Patent Publication (KOKAI) No. 5-341498.
This prior art discloses a method in which if the distance between transparent regions is smaller than a threshold value, a shifter is arranged on one of the transparent regions, and if the shifter arrangement has a conflict, the designer is notified of it. This prior art also discloses an apparatus having an automatic shifter arrangement function, a shifter verification function and a shifter arrangement/verification function for a layout with some shifters already arranged.
Fourth prior art: Japanese Patent Publication (KOKAI) No. 7-13326.
This prior art describes a technique of weighting sides under various conditions where each nodes represents an aperture and each sides represents adjacent relationship of two apertures, and assigning the same phases to nodes at the two ends of at least one side of each closed loop in descending order of weight of sides.
Fifth prior art: "Algorithm for Phase Shift Mask Design with Priority on Shifter Placement", Jpn. J. Appl. Phys. Vol. 32 (1993), PP. 5874-5879, and Japanese Patent Publication (KOKAI) No. 6-308714.
This prior art discloses a method and apparatus for extracting, from an input layout, combinations of two patterns which are adjacent within a designated value, assigning priority in phase shifter arrangement in descending order of length of a side adjacent to the other pattern of the combination, and alternately placing phase shifters in descending order of the priority in shifter arrangement.
Sixth prior art: Japanese Patent Publication (KOKAI) No. 8-328237.
This prior art discloses a method in which, in design data formed from patterns as transparent regions, the adjacent relationship of patterns which are close to each other within a threshold value, all combinations of adjacent patterns are extracted from each adjacent group based on the adjacent relationship, the combinations are sorted in descending order of correction difficulty, the reverse relationship of phase is prepared based on the sorting result, and the phase is assigned based on the prepared reverse relationship.
Seventh prior art: "Computer Aided Design Software for Design Phase-shifting Masks", JPN. J. Appl. Phys. Vol. 32 (1993), pp. 5887-5891.
This paper examines a method of assigning the phase based on the adjacent relationships of patterns, starting from the patterns whose phases has been assigned by designer in advance.
However, these prior arts have the following problems.
The first to third prior arts merely describe the phase assigning method based on a specific pattern as an initial pattern regardless of the presence/absence of phase inconsistency, and no effective shifter arrangement cannot be found even when the phase assignment result is obtained. Therefore, these prior arts cannot realize a shifter arrangement with high resolution.
In the fourth to sixth prior arts, the phase is assigned by weighting in consideration of the length of adjacent sides or the distance between adjacent patterns, so a conflict which is easy to correct can be obtained. However, the resolution is not taken into consideration. For this reason, if there are pattern pairs having almost the same correction easiness but an obvious difference in resolution, phase assignment for salvaging a pattern pair with lower resolution cannot be made. In these prior arts, therefore, no shifter arrangement capable of obtaining high-resolution patterns cannot be realized.
For, e.g., a DRAM, the phase for the memory cell portion is sometimes manually assigned. In this case, the phase of a pattern in the sense amplifier portion or row decoder portion which have not been subjected to phase assignment yet must be assigned without changing the already assigned phase information. From the viewpoint of determining the phase based on a pattern already having a certain phase (fixed phase), each of the first to third prior arts merely describes phase assignment using a specific pattern as an initial pattern, so phase assignment while considering the fixed phase cannot be performed.
The fourth prior art describes a method of minimizing conflicts although this method does not allow phase assignment while considering the fixed phases. In the fifth or sixth prior art, the phase can be assigned in accordance with priority based on the correction easiness although a phase assignment cannot be made in consideration of fixed phases. The seventh prior art takes fixed phases into consideration. However, since this phase assigning method is based on adjacent relationships, phase assignment cannot be made in consideration of resolution easiness or correction easiness at the phase conflict portions.